The vacuum pressure of an electric vacuum interrupter should be monitored in order to maintain the desirable operating characteristic thereof. The superior characteristics of vacuum as a dielectric make its use in power interrupting devices preferable over the use of special arc extinguishing materials, such as gases and liquids. Since vacuum offers both a high dielectric strength and a high recovery rate of voltage per microsecond, interruption can normally be anticipated at the first current zero in an A.C. current waveform. Furthermore, a small gap between contacts can perform the interruption of current. The short gap permits the use of a short contact displacement stroke, enabling the use of a device of low mass and inertia which results in high operating speed and low mechanical shock.
Normally, the operating sequence of the vacuum circuit interrupter from a fault to clear may be accomplished in less than three cycles. Since energy dissipated during a fault is proportional to time, the faster clearing action of a vacuum interrupter means less damage, lower contact erosion, longer maintenance-free contact life, and maximum equipment protection. An important problem in vacuum type electrical devices is that the characteristics of the devices are influenced by the strength of vacuum pressure. Namely, the problem with the use of vacuum circuit interrupters is that if there is a loss of vacuum, as by leakage of air through a crack caused by undue mechanical stresses, both the high strength of the vacuum dielectric and the rapid recovery are lost. The small contact spacing will no longer be able to sustain the high voltages. Arcs and flashovers will occur. The white hot arc will burn the electrode and melt the envelope, and may even extend into and attack other parts of the interrupter assembly.
In power systems it is important to know whether a leak has occurred while the contacts are open or closed during operation of the circuit with which the interrupter is associated. If the leak occurs when the contacts are closed in a grounded three phase system, it is dangerous to interrupt the power systems. Accordingly the power must be turned off upstream of the current interrupter in order to be able to remove and replace the interrupter. If this is not done, an arc will be drawn and the equipment damaged when the interrupter contacts are opened. In recent years various types of pressure measuring systems for vacuum circuit interrupters have been put into practical use. These pressure measuring systems, however, have disadvantages in the practical use.